Are your soybean plant populations too high?

I just finished a study on soybean plant populations that confirmed many of the ideas I have had in the past.

This study was planted on a nonirrigated silt loam soil classified as a Kipling, which is one of the predominant soil types in the “prairie” region of northwest Rankin County, Miss.

It was planted to soybeans on May 6, 2015, enduring an early period of very wet and overcast conditions which lasted until the first week of July, after which the field received very little rain until harvest.

Populations were achieved with a precision planter at rates ranging from 70,000 to 170,000 seeds per acre in 20,000 increments. Seedlings emerged near 90 percent of the planted populations.

The yields were almost the same across all populations, but several other plant characteristics varied according to population.

The lower populations were somewhat shorter in height, and these plants with more space had more vegetative branches, more blooms, more pods, and larger seeds than in the higher populations. These factors generally account for the fact that the lower populations produced yields equal to the higher populations.

Those familiar with mapping cotton will recognize the similarity in soybeans.

An apparent question for anyone who followed this study in the field or who may now be reading this brief summary is how much the weeks of dry weather affected yield? I have concerns about this myself, but there are other facts that tend to make me believe the result might have been similar under more “ideal” conditions.

To begin with, the green weight of plants in mid-July before the onset of drought stress indicated that plants in the lower populations were producing much more biomass (weight) than those in the higher populations. These plants with more space had a higher chlorophyll index, suggesting that the rate of photosynthesis was higher.

At the end of the first week of August, when drought stress had begun to affect the crop visually, the lower populations showed much higher transpiration rates than the more dense populations, indicating that these plants still had adequate moisture while the thicker stands were already severely stressed.

These pre-harvest data suggest to me that the lower populations were performing better well before the onset of drought.

The natural question is how low can we go with soybean plant populations without affecting yield? The answer suggested by this study is 85,000 to 100,000 plants, which were produced by planting from 90,000 to 110,000 seeds per acre. Since these seeds were excellent quality —germinating near 90 percent — a grower may want to hedge and move up to the 100,000 to 110,000 level of planting seed with good seed treatments.

I hope to repeat this study on an irrigated site in 2016 to test my belief that the trends will be similar. If we can “safely” reduce seeding rates by 25 percent or more — which is suggested by this study — farmers can reduce production costs and increase profits without reducing yield.

Some have already adopted this level of seeding successfully, but if there are doubts, the best idea is to try a few acres before changing completely.

Variable seeding rates may make this even simpler by enabling growers to use populations that fit a wide range of field conditions.